In semiconductor manufacturing charged particle processing tools, such as ion implantation, plasma etch, plasma deposition, or charged particle sputtering, are used to manufacture integrated circuits. Today's integrated circuit devices use relatively low voltages (e.g. 5 or 12 Volt logic are common). An adverse effect of the use of charge particle process tools which negatively impacts production yield is created because the charge particle processes inherently create potential differences across the process surface, or between surface and substrate. Potential differences only a few times the nominal voltage level can cause destructive breakdown of thin dielectrics or premature wearout of MOS transistors, if sufficient charge is available. For example, 30 Volts can easily destroy nominal 10 Volt devices. Moreover, even if yield does not seem negatively impacted by testing at the time of manufacture, charging damage can be manifested over time in that the effects do not appear until the integrated circuit has been used normally. Thus, an integrated circuit subject to excess electrical steps during processing may not be reliable.
Charging has traditionally been a problem for ion implantation because implant beams can easily create 30 Volt potential differences at the target substrate surface. Traditional solutions are to reduce implant current (which limits throughput), or to use an electron gun to neutralize beam potentials.
Alternatively, have recently begun ion beam implant vendors have begun to introduce plasma flood systems near the target. A plasma is an ionized gas, i.e., a gas in which some atoms or molecules have been stripped of at least one electron to create ions. Plasmas are quasi-neutral, which means that the density of ions and electrons is very nearly equal. Plasmas diffused near the beam and/or target surface are effective for controlling charging damage to integrated circuits during ion implantation because the presence of the plasma ions insures that a sufficient density of electrons is available to short out significant large scale potential differences.
However, as integrated circuit technology moves to ever smaller, faster transistors, the intrinsic threshold voltage decreases due to thinner dielectrics. For example, 3 Volt logic is currently being introduced, and 1.5 Volts is anticipated in the future. Lower logic levels means that previous damage thresholds will not be tolerated by the integrated circuit devices. Manufacturing tools which meet the lower thresholds for charging damage to integrated circuits will be required.
A plasma flood system of the prior art which causes plasma to be introduced to the process surface can itself bias the process surface, because of the energetic electrons associated with the generation of that plasma. In a typical plasma, the electron temperature is at least a few electron Volts, because energetic primary electrons (20 to 100 Volts) are needed to ionize the gas. Primaries, that is, energetic electrons, have the effect of raising the average electron energy, i.e. electron temperature, of the plasma. Moreover, raising the plasma density raises the required power at least proportionally, which can also increase the electron temperature, as well as the floating potential, of a surface which is exposed to the plasma.
In the generation of plasma, the natural potential of a small conductor exposed to the plasma is related to the energy of all electrons which can reach the conductor. The inherent floating potential of a plasma scales as a multiple (often 3 to 8) of the electron temperature. Thus, a plasma having a lower electron temperature which is achieved by removing the primary electrons, will have a lower characteristic floating potential than a plasma with primary electrons and a higher temperature.
A carefully designed magnetic field can separate the electron components of the plasma into a high electron temperature, plasma generation region, and a low electron temperature cold plasma region. In recent years, this technique has been given a name "magnetic filter."
A magnetic filter has a magnetic field just strong enough to confine the energetic primary electrons to the generation region, but weak enough to allow ions to cross. Low energy collisional electrons follow the ions, cross the filter to the cold region and maintain quasi-neutrality. Thus, ions in the cold regions are neutralized by collisional cold electrons. A plasma made up of ions and low temperature electrons is known as a "cold plasma." For example, see U.S. Pat. No. 4,447,732 issued to K. Leung et al. on May 8, 1984 for a particular type of magnetic filter used in conjunction with an ion beam accelerator wherein a specific magnetic design is used to produce a particular ion species for use in an ion beam.